Electro-optic display and adhesive composition for use therein

ABSTRACT

An electro-optic display comprises first and second substrates, and an adhesive layer and a layer of electro-optic material disposed between the first and second substrates. The adhesive layer has a volume resistivity in the range of about 10 9  to about 10 11  ohm cm and comprises a mixture of an adhesive material having a volume resistivity of at least about 5×10 11  ohm cm and a filler having a volume resistivity not less than about 10 7  ohm cm, the filler being present in the mixture in a proportion above its percolation threshold in the adhesive material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Provisional Application Serial No.60/304,015, filed Jul. 9, 2001; the entire disclosure of thisProvisional Application is herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to electro-optic displays and to adhesivecompositions for use therein. More specifically, this invention relatesto adhesive compositions having controlled volume resistivity, and toelectro-optic displays incorporating such adhesive compositions. Theadhesive compositions of the present invention may also be useful inapplications other than electro-optic displays.

Electro-optic displays comprise a layer of electro-optic material, aterm which is used herein in its conventional meaning in the art torefer to a material having first and second display states differing inat least one optical property, the material being changed from its firstto its second display state by application of an electric field to thematerial. The optical property is typically color perceptible to thehuman eye, but may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

One type of electro-optic display is the rotating bichromal member typeas described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782 and5,760,761 (this type of electro-optic medium is often referred to as a“rotating bichromal ball” medium, but the term “rotating bichromalmember” is preferred since in some versions of the medium the rotatingmembers are not spherical).

Another type of electro-optic medium is an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). Nanochromic films of this type are also described, forexample, in International Applications Publication Nos. WO 98/35267 andWO 01/27690, and in copending Applications Serial Nos. 60/365,368;60/365,369; 60/365,385 and 60/365,365, all filed Mar. 18, 2002, andApplication Serial Nos. 60/319,279; 60/319,280; and 60/319,281, allfiled May 31, 2002; the entire contents of all these applications areherein incorporated by reference.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspension medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,241,921; 6,249,271;6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971;6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; and 6,392,786;U.S. patent application Publication Nos. 2001-0045934; 2002-0018042;2002-0019081; and 2002-0021270; and International ApplicationsPublication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO98/41899; WO 99/10767; WO 99/10768; WO 99/10769; WO 99/47970; WO99/53371; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/67678; WO00/03349; WO 00/03291; WO 00/05704; WO 00/20921; WO 00/20922; WO00/20923; WO 00/26761; WO 00/36465; WO 00/36560; WO 00/36666; WO00/38000; WO 00/38001; WO 00/59625; WO 00/60410; WO 00/67110; WO00/67327 WO 01/02899; WO 01/07691; WO 01/08241; WO 01/08242; WO01/17029; WO 01/17040; WO 01/17041; WO 01/80287 and WO 02/07216. Theentire disclosures of all these patents and published applications areherein incorporated by reference.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display in whichthe electrophoretic medium comprises a plurality of discrete droplets ofan electrophoretic fluid and a continuous phase of a polymeric material,and that the discrete droplets of electrophoretic fluid within such apolymer-dispersed electrophoretic display may be regarded as capsules ormicrocapsules even though no discrete capsule membrane is associatedwith each individual droplet; see for example, WO 01/02899, at page 10,lines 6-19. See also copending application Ser. No. 09/683,903, filedFeb. 28, 2002, the entire disclosure of which is herein incorporated byreference, and the corresponding International ApplicationPCT/US02/06393.

An encapsulated, electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic materials, for example, liquid crystal,especially polymer-dispersed liquid crystal, may also be used in thedisplays of the present invention.

In addition to the layer of electro-optic material, an electro-opticdisplay normally comprises at least two other layers disposed on opposedsides of the electro-optic material, one of these two layers being anelectrode layer. In most such displays both the layers are electrodelayers, and one or both of the electrode layers are patterned to definethe pixels of the display. For example, one electrode layer may bepatterned into elongate row electrodes and the other into elongatecolumn electrodes running at right angles to the row electrodes, thepixels being defined by the intersections of the row and columnelectrodes. Alternatively, and more commonly, one electrode layer hasthe form of a single continuous electrode and the other electrode layeris patterned into a matrix of pixel electrodes, each of which definesone pixel of the display. In another type of electro-optic display,which is intended for use with a stylus, print head or similar movableelectrode separate from the display, only one of the layers adjacent theelectro-optic layer comprises an electrode, the layer on the opposedside of the electro-optic layer typically being a protective layerintended to prevent the movable electrode damaging the electro-opticlayer.

The manufacture of a three-layer electro-optic display normally involvesat least one lamination operation. For example, in several of theaforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide or a similar conductive coating (which acts as an oneelectrode of the final display) on a plastic film, the capsules/bindercoating being dried to form a coherent layer of the electrophoreticmedium firmly adhered to the substrate. Separately, a backplane,containing an array of pixel electrodes and an appropriate arrangementof conductors to connect the pixel electrodes to drive circuitry, isprepared. To form the final display, the substrate having thecapsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display useable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. The obviouslamination technique for mass production of displays by the processdescribed above is roll lamination using a lamination adhesive, whichmay be a pressure sensitive adhesive (PSA). Roll lamination isespecially desirable since the pressures used in such lamination areeffective in expelling air from between the two materials beinglaminated, thus avoiding unwanted air bubbles in the final display; suchair bubbles may introduce undesirable artifacts in the images producedon the display.

A variety of lamination adhesives are of course readily availablecommercially, and several of these commercial materials have been foundto meet the usual physico-chemical requirements for use in the rolllamination process described above, in that they provide a bond ofsufficient strength and durability and are chemically compatible withthe other components of the display, in that, for example they do notrelease any materials which interfere with the proper switching of theelectrophoretic medium. Unfortunately, there is one requirement for theproper operation of electrophoretic and other electro-optic displayswhich has not been found to be met by any commercial pressure sensitiveadhesive, namely appropriate volume resistivity. In an electro-opticdisplay, it is highly desirable to correlate the volume resistivity ofthe adhesive layer with that of the electro-optic medium itself. If theresistivity of the adhesive layer is too high, a substantial voltagedrop will occur within the adhesive layer, requiring an increase involtage across the electrodes to maintain the same voltage drop acrossthe electro-optic medium, and thus maintain the same switching time ofthe medium. Increasing the voltage across the electrodes in this manneris undesirable, since it increases the power consumption of the display,and may require the use of more complex and expensive control circuitryto generate (in the case of battery-powered portable devices) and/orhandle the increased voltage involved. On the other hand, if theadhesive layer, which extends continuously across the display, is incontact with a matrix of closely-spaced electrodes, such as the pixelelectrode arrays used for active matrix displays, the volume resistivityof the adhesive layer should not be too low, or lateral conduction ofelectric current through the continuous adhesive layer may causeundesirable cross-talk between adjacent electrodes.

Encapsulated electrophoretic media typically have volume resistivitiesof the order of 5×10¹⁰ ohm cm, and the volume resistivities of otherelectro-optic media are usually of the same order of magnitude. For theforegoing reasons, the volume resistivity of lamination adhesives usedwith such media should be around 10¹⁰ ohm cm, whereas commercial PSA”shave volume resistivities of about 10¹² to 10¹⁵ ohm cm. It is known thatthe volume resistivities of insulators (and lamination adhesives areinsulators by normal standards) can be decreased by blending aconductive filler (for example, silver flake, carbon black, polyanilineor poly(3,4-ethylenedioxythiophene)) with the insulator. Unfortunately,there are grave difficulties in adopting this approach to achieve thevolume resistivities of about 10¹⁰ ohm cm required for adhesives used inelectro-optic displays. Experimentally, it has been found that as theproportion of filler in the adhesive is gradually increased from zero,the volume resistivity of the blend remains substantially equal to thatof the adhesive until a threshold value (known as the percolationthreshold) is reached. At this point, a small increase in the proportionof filler present produces an enormous decrease in resistivity byseveral orders of magnitude to a value typically about one order ofmagnitude greater than that of the conductive filler itself. This isillustrated in FIG. 1 of the accompanying drawings, which is taken fromDonnet, J-B., et al. (eds.), Carbon Black Science and Technology (2dEdn., Marcel Dekker, New York, 1993), which illustrates the effect ofadding various types of carbon black to high density polyethylene. ThisFigure shows that the percolation threshold may vary over a wide rangefrom less than 5 percent to more than 50 percent. After this sudden dropin resistivity, further increases in the proportion of conductive fillercause only a very gradual further decrease in volume resistivity of theblend. If one starts from an adhesive having a volume resistivity ofabout 10¹² to 10¹⁵ ohm cm, the desired volume resistivity of about 10¹⁰ohm cm required for adhesives used in electro-optic displays typicallyfalls in the middle of the very steep section of thecomposition/resistivity curve, where a very small change in theproportion of filler may cause an order of magnitude change in volumeresistivity. It is difficult (and may be impracticable under massproduction conditions) to control the proportion of filler withsufficient accuracy to maintain the volume resistivity desired in theadhesive. Furthermore, the variation of resistivity with proportion offiller at this point is so vertiginous that inevitable slight variationsin the proportion of filler in particular areas of the display may causeundesirable effects in the display.

It has now been discovered that the volume resistivity of adhesives canbe varied in a controlled manner to the range required for use inelectro-optic displays be blending the adhesive with a filler having acontrolled volume resistivity. The volume resistivity of this fillershould not be lower that about two orders of magnitude less than theintended volume resistivity of the final blend. Desirably, the volumeresistivity of the filler should be at least an order of magnitude lessthat the intended volume resistivity of the final blend.

SUMMARY OF THE INVENTION

Accordingly, in one aspect this invention provides an electro-opticdisplay comprising first and second substrates, and an adhesive layerand a layer of electro-optic material disposed between the first andsecond substrates, the adhesive layer having a volume resistivity in therange of about 10⁹ to about 10¹¹ ohm cm and comprising a mixture of anadhesive material having a volume resistivity of at least about 5×10¹¹ohm cm and a filler having a volume resistivity not less than about 10⁷ohm cm, the filler being present in the mixture in a proportion aboveits percolation threshold in the adhesive material.

This invention also provides an adhesive composition comprising amixture of an adhesive material having a volume resistivity of at leastabout 5×10¹¹ ohm cm and a filler having a volume resistivity not lessthan about 10⁷ ohm cm, the filler being present in the mixture in aproportion above its percolation threshold in the adhesive material.

This invention also provides a method for the preparation of an adhesivecomposition having a volume resistivity of X (where X is an arbitrarynumber) starting from an adhesive material having a volume resistivityof at least about 10X, this method comprising blending the adhesivematerial with a filler having a volume resistivity not greater thanabout 0.1X but not less than about 10⁻³X, the filler being blended withthe adhesive material in a proportion above its percolation threshold inthe adhesive material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 of the accompanying drawings is a graph showing the change involume resistivity of polyethylene upon addition of varying proportionsof carbon black thereto;

FIG. 2 is a graph showing the variation in volume resistivity ofGME-2417 (see below) upon addition of varying proportions of Soluol105-304 (see below), as described in Example 1 below; and

FIG. 3 is a graph showing the variation in volume resistivity ofGME-2417 (see below) upon addition of varying proportions of NeoRez R9320 (see below), as described in Example 2 below.

DETAILED DESCRIPTION

As already mentioned, this invention provides an adhesive compositioncomprising a mixture of an adhesive material having a volume resistivityof at least about 5×10¹¹ ohm cm and a filler having a volume resistivitynot less than about 10⁷ ohm cm, the filler being present in the mixturein a proportion above its percolation threshold in the adhesivematerial, and an electro-optic display incorporating this adhesivecomposition. The volume resistivity of the present adhesive compositioncan be varied in a highly controllable manner by varying the proportionof the filler, thus allowing this volume resistivity to be adjusted toits optimum value for use with any specific electro-optic medium andother components of a display.

The present invention is especially applicable to pressure sensitiveadhesives, although it is also applicable to other types of adhesive,such as hot melt and heat-activated adhesives. The PSA's used in thepresent invention may be water-based, solvent-based or 100 percentsolids, such as ultra-violet curable PSA's, and may be, for example,acrylate, urethane or silicone-based PSA's. Specific commercialmaterials which have been found to be useful in the present inventioninclude Gelva (Registered Trade Mark) multipolymer resin emulsion 2417(usually known as GME-2417), an acrylic multipolymer emulsion availablefrom Solutia Inc, 10300 Olive Boulevard, P.O. Box 66760, St. Louis Mo.63166-6760, and Reichhold 97982-01, an acrylic-based PSA with a glasstransition temperature of about −45° C., available from Reichhold, Inc.,P.O. Box 13582, Research Triangle Park N.C. 27709-3582.

The filler used in the present invention is typically a material havinga volume resistivity in the range of about 10⁸ to 5×10⁹ ohm cm, andsuitable materials include polyethylene glycol, urethane polymerdispersions, tailored acrylate polymers, diamond, zirconium oxide,silicon carbide and sodium-doped glass. The presently preferred filleris Soluol 21-105HS, a polyurethane available from Soluol Chemical Co.,Inc., P.O. Box 112, West Warwick R.I. 02893; Soluol 105-304 from thesame manufacturer has also been found useful. The percolation thresholdfor the filler varies widely, from greater than 20 to less than 2percent by volume, but can readily be determined empirically for anyspecific adhesive material/filler combination; as illustrated in theExamples below, the variation of volume resistivity with proportion offiller in the adhesive material/filler blend has the same general formas shown in FIG. 1, with a sharp and characteristic decrease in volumeresistivity at the percolation threshold. Thus, the proportion of fillerin the adhesive layer will typically be in the range of about 3 to about50 percent by weight. The presently preferred blend for use inlaminating electro-optic displays comprises about 20 percent by weight(calculated on dry weight) of Soluol 21-105HS filler and 80 percent byweight Reichhold 97982-01 pressure sensitive adhesive.

The electro-optic medium present in the displays of the presentinvention may be of any of the types previously discussed. Thus, theelectro-optic medium may be a rotating bichromal member orelectrochromic medium. However, it is preferred that the electro-opticmedium be an electrophoretic medium comprising a plurality of capsules,each capsule comprising a capsule wall and an internal phase comprisingelectrically charged particles suspended in a suspending fluid andcapable of moving through the fluid on application of an electric fieldto the electrophoretic medium. Desirably, in addition to the capsules,the electrophoretic medium comprises a polymeric binder within which thecapsules are held.

Also, as already indicated, the display may be of any of the formsdescribed in the aforementioned patents and applications. Thus,typically the display will comprise at least one electrode disposedbetween the electro-optic medium and one of the substrates, thiselectrode being arranged to apply an electric field to the electro-opticmedium. Generally, the display will comprise two electrodes disposed onopposed sides of the electro-optic medium and between the electro-opticmedium and the two substrates, at least one of the electrodes and theadjacent substrate being light-transmissive such that the electro-opticmedium can be viewed through the light-transmissive substrate andelectrode.

The following Examples are now given, though by way of illustrationonly, to show details of preferred materials, processes and techniquesused in the present invention.

EXAMPLE 1

This Example illustrates the variation in volume resistivity achieved byadding the aforementioned Soluol 105-304 to the aforementioned GME-2417.

Varying proportions of Soluol 105-304 were blended with GME-2417 and thevolume resistivities of films made from the resultant blends werecalculated after measuring the current flow through a film of knowndimensions on the application of a known voltage. The results areillustrated in FIG. 2 of the accompanying drawings, which also showstheoretical results calculated assuming a percolation threshold for theSoluol 105-304 of 25% by volume. A simple model for percolation behaviorassumes that for filler loadings below the percolation threshold theresistivity of a blend is given by the volume-fraction-weightedresistivity of the adhesive material in series with the filler. In otherwords, before percolation:

 VR _(blend)=ρ_(adhesive)φ_(adhesive)+ρ_(filler)φ_(filler)

where ρ is the volume resistivity and φ is the volume fraction of eachmaterial in the film. Blends with a filler loading above the percolationthreshold behave electrically as two resistors in parallel, and the filmresistivity can be estimated by:${V\quad R_{b\quad l\quad e\quad n\quad d}} = \frac{1}{\left( {\frac{\varphi_{adhesive}}{\rho_{adhesive}} + \frac{\varphi_{filler}}{\rho_{filler}}} \right)}$

The open circles in FIG. 2 (and those in FIG. 3 discussed below) arevolume resistivities calculated using these equations.

It will be seen from FIG. 2 that this model fits the experimental datareasonably well. It will also be seen that at approximately 40 percentby volume filler loading, this blend has a volume resistivity ofapproximately 10⁹ ohm cm, and is thus useful for laminating anelectro-optic display.

EXAMPLE 2

This Example illustrates the variation in volume resistivity achieved byadding NeoRez R 9320 (a water-dispersible polyurethane available fromNeoResins, 730 Main Street, Wilmington, Mass. 01887) to theaforementioned GME-2417.

Varying proportions of NeoRez R 9320 were blended with GME-2417 and thevolume resistivities of the resultant blends were measured in the sameway as in Example 1 above. The results are illustrated in FIG. 3 of theaccompanying drawings, which also shows theoretical results calculatedassuming a percolation threshold of 20 percent by volume. As in Example1 above, the experimental results are a reasonably good fit to themodel, displaying a sudden drop as the percolation threshold is passedand a slow decrease thereafter as the proportion of filler is increased.It will also be seen that at about 40 percent filler loading, the blendhas a volume resistivity of about 3×10¹⁰ ohm cm, rendering it suitablefor use in lamination of an electro-optic display.

Apart from the inclusion of the adhesive composition of the presentinvention, the electrophoretic media and displays of the presentinvention may employ the same components and manufacturing techniques asin the aforementioned patents and applications. The following SectionsA-E describe useful materials for use in the various components of theencapsulated electrophoretic displays of the present invention.

A. Electrophoretic Particles

There is much flexibility in the choice of particles for use inelectrophoretic displays, as described above. For purposes of thisinvention, a particle is any component that is charged or capable ofacquiring a charge (i.e., has or is capable of acquiring electrophoreticmobility), and, in some cases, this mobility may be zero or close tozero (i.e., the particles will not move). The particles may be neatpigments, dyed (laked) pigments or pigment/polymer composites, or anyother component that is charged or capable of acquiring a charge.Typical considerations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light. The particles for use in the invention may furtherinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective, such as corner cubes,or they may be electroluminescent, such as zinc sulfide particles, whichemit light when excited by an AC field, or they may be photoluminescent.Zinc sulfide electroluminescent particles may be encapsulated with aninsulative coating to reduce electrical conduction. Finally, theparticles may be surface treated so as to improve charging orinteraction with a charging agent, or to improve dispersability.

One particle for use in electrophoretic displays of the invention istitania. The titania particles may be coated with a metal oxide, such asaluminum oxide or silicon oxide, for example. The titania particles mayhave one, two, or more layers of metal-oxide coating. For example, atitania particle for use in electrophoretic displays of the inventionmay have a coating of aluminum oxide and a coating of silicon oxide. Thecoatings may be added to the particle in any order.

The electrophoretic particle is usually a pigment, a polymer, a lakedpigment, or some combination of the above. A neat pigment can be anypigment, and, usually for a light colored particle, pigments such asrutile (titania), anatase (titania), barium sulfate, kaolin, or zincoxide are useful. Some typical particles have high refractive indices,high scattering coefficients, and low absorption coefficients. Otherparticles are absorptive, such as carbon black or colored pigments usedin paints and inks. The pigment should also be insoluble in thesuspending fluid. Yellow pigments such as diarylide yellow, Hansayellow, and benzidin yellow have also found use in similar displays. Anyother reflective material can be employed for a light colored particle,including non-pigment materials, such as metallic particles.

Useful neat pigments include, but are not limited to, PbCrO₄, Cyan blueGT 55-3295 (American Cyanamid Company, Wayne, N.J.), Cibacron Black BG(Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba),Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL(Ciba), Acetamine Black, CBS (E. I. du Pont de Nemours and Company,Inc., Wilmington, Del., hereinafter abbreviated “du Pont”), CroceinScarlet N Ex (du Pont) (27290), Fiber Black VF (du Pont) (30235), LuxolFast Black L (du Pont) (Solv. Black 17), Nirosine Base No. 424 (du Pont)(50415 B), Oil Black BG (du Pont) (Solv. Black 16), Rotalin Black RM (duPont), Sevron Brilliant Red 3 B (du Pont); Basic Black DSC (DyeSpecialties, Inc.), Hectolene Black (Dye Specialties, Inc.), AzosolBrilliant Blue B (GAF, Dyestuff and Chemical Division, Wayne, N.J.)(Solv. Blue 9), Azosol Brilliant Green BA (GAF) (Solv. Green 2), AzosolFast Brilliant Red B (GAF), Azosol Fast Orange RA Conc. (GAF) (Solv.Orange 20), Azosol Fast Yellow GRA Conc. (GAF) (13900 A), Basic BlackKMPA (GAF), Benzofix Black CW-CF (GAF) (35435), Cellitazol BNFV ExSoluble CF (GAF) (Disp. Black 9), Celliton Fast Blue AF Ex Conc (GAF)(Disp. Blue 9), Cyper Black IA (GAF) (Basic Black 3), Diamine Black CAPEx Conc (GAF) (30235), Diamond Black EAN Hi Con. CF (GAF) (15710),Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EA Ex CF (GAF)(30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBK Powd. (GAF)(59850), Indocarbon CLGS Conc. CF (GAF) (53295), Katigen Deep Black NNDHi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF) (Azoic Black 4);Sulphone Cyanine Black BA-CF (GAF) (26370), Zambezi Black VD Ex Conc.(GAF) (30015); Rubanox Red CP-1495 (The Sherwin-Williams Company,Cleveland, Ohio) (15630); Raven 11 (Columbian Carbon Company, Atlanta,Ga.), (carbon black aggregates with a particle size of about 25 μm),Statex B-12 (Columbian Carbon Co.) (a furnace black of 33 μm averageparticle size), and chrome green.

Particles may also include laked, or dyed, pigments. Laked pigments areparticles that have a dye precipitated on them or which are stained.Lakes are metal salts of readily soluble anionic dyes. These are dyes ofazo, triphenylmethane or anthraquinone structure containing one or moresulphonic or carboxylic acid groupings. They are usually precipitated bya calcium, barium or aluminum salt onto a substrate. Typical examplesare peacock blue lake (Cl Pigment Blue 24) and Persian orange (lake ofCl Acid Orange 7), Black M Toner (GAF) (a mixture of carbon black andblack dye precipitated on a lake).

A dark particle of the dyed type may be constructed from any lightabsorbing material, such as carbon black, or inorganic black materials.The dark material may also be selectively absorbing. For example, a darkgreen pigment may be used. Black particles may also be formed bystaining latices with metal oxides, such latex copolymers consisting ofany of butadiene, styrene, isoprene, methacrylic acid, methylmethacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.Useful sizes may range from 1 μm up to about 100 μm, as long as theparticles are smaller than the bounding capsule. The density of theelectrophoretic particle may be substantially matched to that of thesuspending (i.e., electrophoretic) fluid. As defined herein, asuspending fluid has a density that is “substantially matched” to thedensity of the particle if the difference in their respective densitiesis between about zero and about two grams/milliliter (“g/ml”). Thisdifference is preferably between about zero and about 0.5 g/ml.

Useful polymers for the particles include, but are not limited to:polystyrene, polyethylene, polypropylene, phenolic resins, du Pont Elvaxresins (ethylene-vinyl acetate copolymers), polyesters, polyacrylates,polymethacrylates, ethylene acrylic acid or methacrylic acid copolymers(Nucrel Resins du Pont, Primacor Resins Dow Chemical), acryliccopolymers and terpolymers (Elvacite Resins du Pont) and PMMA. Usefulmaterials for homopolymer/pigment phase separation in high shear meltinclude, but are not limited to, polyethylene, polypropylene,poly(methyl methacrylate), poly(isobutyl methacrylate), polystyrene,polybutadiene, polyisoprene, polyisobutylene, poly(lauryl methacrylate),poly(stearyl methacrylate), poly(isobornyl methacrylate), poly(t-butylmethacrylate), poly(ethyl methacrylate), poly(methyl acrylate),poly(ethyl acrylate), polyacrylonitrile, and copolymers of two or moreof these materials. Some useful pigment/polymer complexes that arecommercially available include, but are not limited to, Process MagentaPM 1776 (Magruder Color Company, Inc., Elizabeth, N.J.), Methyl VioletPMA VM6223 (Magruder Color Company, Inc., Elizabeth, N.J.), and NaphtholFGR RF6257 (Magruder Color Company, Inc., Elizabeth, N.J.).

The pigment-polymer composite may be formed by a physical process,(e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. For example, the processes andmaterials for both the fabrication of liquid toner particles and thecharging of those particles may be relevant.

New and useful electrophoretic particles may still be discovered, but anumber of particles already known to those skilled in the art ofelectrophoretic displays and liquid toners can also prove useful. Ingeneral, the polymer requirements for liquid toners and encapsulatedelectrophoretic inks are similar, in that the pigment or dye must beeasily incorporated therein, either by a physical, chemical, orphysicochemical process, may aid in the colloidal stability, and maycontain charging sites or may be able to incorporate materials whichcontain charging sites. One general requirement from the liquid tonerindustry that is not shared by encapsulated electrophoretic inks is thatthe toner must be capable of “fixing” the image, i.e., heat fusingtogether to create a uniform film after the deposition of the tonerparticles.

Typical manufacturing techniques for particles may be drawn from theliquid toner and other arts and include ball milling, attrition, jetmilling, etc. The process will be illustrated for the case of apigmented polymeric particle. In such a case the pigment is compoundedin the polymer, usually in some kind of high shear mechanism such as ascrew extruder. The composite material is then (wet or dry) ground to astarting size of around 10 μm. It is then dispersed in a carrier liquid,for example ISOPAR® (Exxon, Houston, Tex.), optionally with some chargecontrol agent(s), and milled under high shear for several hours down toa final particle size and/or size distribution.

Another manufacturing technique for particles is to add the polymer,pigment, and suspending fluid to a media mill. The mill is started andsimultaneously heated to a temperature at which the polymer swellssubstantially with the solvent. This temperature is typically near 100°C. In this state, the pigment is easily encapsulated into the swollenpolymer. After a suitable time, typically a few hours, the mill isgradually cooled back to ambient temperature while stirring. The millingmay be continued for some time to achieve a small enough particle size,typically a few microns in diameter. The charging agents may be added atthis time. Optionally, more suspending fluid may be added.

Chemical processes such as dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization precipitation,phase separation, solvent evaporation, in situ polymerization, seededemulsion polymerization, or any process which falls under the generalcategory of microencapsulation may be used. A typical process of thistype is a phase separation process wherein a dissolved polymericmaterial is precipitated out of solution onto a dispersed pigmentsurface through solvent dilution, evaporation, or a thermal change.Other processes include chemical means for staining polymeric latices,for example with metal oxides or dyes.

B. Suspending Fluid

The suspending fluid containing the particles can be chosen based onproperties such as density, refractive index, and solubility. Apreferred suspending fluid has a low dielectric constant (about 2), highvolume resistivity (about 10¹⁵ ohm cm), low viscosity (less than 5centistokes (“cst”)), low toxicity and environmental impact, low watersolubility (less than 10 parts per million (“ppm”)), high specificgravity (greater than 1.5), a high boiling point (greater than 90° C.),and a low refractive index (less than 1.2).

The choice of suspending fluid may be based on concerns of chemicalinertness, density matching to the electrophoretic particle, or chemicalcompatibility with both the electrophoretic particle and boundingcapsule. The viscosity of the fluid should be low when movement of theparticles is desired. The refractive index of the suspending fluid mayalso be substantially matched to that of the particles. As used herein,the refractive index of a suspending fluid is substantially matched tothat of a particle if the difference between their respective refractiveindices is between about zero and about 0.3, and is preferably betweenabout 0.05 and about 0.2.

Additionally, the fluid may be chosen to be a poor solvent for somepolymers, which is advantageous for use in the fabrication ofmicroparticles, because it increases the range of polymeric materialsuseful in fabricating particles of polymers and pigments. Organicsolvents, such as halogenated organic solvents, saturated linear orbranched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are some useful suspending fluids. Thesuspending fluid may comprise a single fluid. The fluid will, however,often be a blend of more than one fluid in order to tune its chemicaland physical properties. Furthermore, the fluid may contain surfacemodifiers to modify the surface energy or charge of the electrophoreticparticle or bounding capsule. Reactants or solvents for themicroencapsulation process (oil soluble monomers, for example) can alsobe contained in the suspending fluid. Charge control agents can also beadded to the suspending fluid.

Useful organic solvents include, but are not limited to, epoxides, suchas decane epoxide and dodecane epoxide; vinyl ethers, such as cyclohexylvinyl ether and Decave® (International Flavors & Fragrances, Inc., NewYork, N.Y.); and aromatic hydrocarbons, such as toluene and naphthalene.Useful halogenated organic solvents include, but are not limited to,tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene and carbontetrachloride. These materials have high densities. Useful hydrocarbonsinclude, but are not limited to, dodecane, tetradecane, the aliphatichydrocarbons in the Isopar® series (Exxon, Houston, Tex.), Norpar® (aseries of normal paraffinic liquids), Shell-Sol® (Shell, Houston, Tex.),and Sol-Trol® (Shell), naphtha, and other petroleum solvents. Thesematerials usually have low densities. Useful examples of silicone oilsinclude, but are not limited to, octamethyl cyclosiloxane and highermolecular weight cyclic siloxanes, poly(methyl phenyl siloxane),hexamethyldisiloxane, and polydimethylsiloxane. These materials usuallyhave low densities. Useful low molecular weight halogen-containingpolymers include, but are not limited to, poly(chlorotrifluoroethylene)polymer (Halogenated Hydrocarbon Inc., River Edge, N.J.), Galden® (aperfluorinated ether from Ausimont, Morristown, N.J.), or Krytox® fromdu Pont (Wilmington, Del.). In a preferred embodiment, the suspendingfluid is a poly(chlorotrifluoroethylene) polymer. In a particularlypreferred embodiment, this polymer has a degree of polymerization fromabout 2 to about 10. Many of the above materials are available in arange of viscosities, densities, and boiling points.

The fluid must be capable of being formed into small droplets prior to acapsule being formed. Processes for forming small droplets includeflow-through jets, membranes, nozzles, or orifices, as well asshear-based emulsifying schemes. The formation of small drops may beassisted by electrical or sonic fields. Surfactants and polymers can beused to aid in the stabilization and emulsification of the droplets inthe case of an emulsion type encapsulation. One surfactant for use indisplays of the invention is sodium dodecylsulfate.

It can be advantageous in some displays for the suspending fluid tocontain an optically absorbing dye. This dye must be soluble in thefluid, but will generally be insoluble in the other components of thecapsule. There is much flexibility in the choice of dye material. Thedye can be a pure compound, or blends of dyes to achieve a particularcolor, including black. The dyes can be fluorescent, which would producea display in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable by, for example, thermal, photochemical orchemical diffusion processes, forming a solid absorbing polymer insidethe bounding shell.

There are many dyes that can be used in encapsulated electrophoreticdisplays. Properties important here include light fastness, solubilityin the suspending liquid, color, and cost. These dyes are generallychosen from the classes of azo, anthraquinone, and triphenylmethane typedyes and may be chemically modified so as to increase their solubilityin the oil phase and reduce their adsorption by the particle surface.

A number of dyes already known to those skilled in the art ofelectrophoretic displays will prove useful. Useful azo dyes include, butare not limited to: the Oil Red dyes, and the Sudan Red and Sudan Blackseries of dyes. Useful anthraquinone dyes include, but are not limitedto: the Oil Blue dyes, and the Macrolex Blue series of dyes. Usefultriphenylmethane dyes include, but are not limited to, Michler's hydrol,Malachite Green, Crystal Violet, and Auramine O.

C. Charge Control Agents and Particle Stabilizers

Charge control agents are used to provide good electrophoretic mobilityto the electrophoretic particles. Stabilizers are used to preventagglomeration of the electrophoretic particles, as well as prevent theelectrophoretic particles from irreversibly depositing onto the capsulewall. Either component can be constructed from materials across a widerange of molecular weights (low molecular weight, oligomeric, orpolymeric), and may be a single pure compound or a mixture. The chargecontrol agent used to modify and/or stabilize the particle surfacecharge is applied as generally known in the arts of liquid toners,electrophoretic displays, non-aqueous paint dispersions, and engine-oiladditives. In all of these arts, charging species may be added tonon-aqueous media in order to increase electrophoretic mobility orincrease electrostatic stabilization. The materials can improve stericstabilization as well. Different theories of charging are postulated,including selective ion adsorption, proton transfer, and contactelectrification.

An optional charge control agent or charge director may be used. Theseconstituents typically consist of low molecular weight surfactants,polymeric agents, or blends of one or more components and serve tostabilize or otherwise modify the sign and/or magnitude of the charge onthe electrophoretic particles. The charging properties of the pigmentitself may be accounted for by taking into account the acidic or basicsurface properties of the pigment, or the charging sites may take placeon the carrier resin surface (if present), or a combination of the two.Additional pigment properties which may be relevant are the particlesize distribution, the chemical composition, and the lightfastness.

Charge adjuvants may also be added. These materials increase theeffectiveness of the charge control agents or charge directors. Thecharge adjuvant may be a polyhydroxy compound or an aminoalcoholcompound, and is preferably soluble in the suspending fluid in an amountof at least 2% by weight. Examples of polyhydroxy compounds whichcontain at least two hydroxyl groups include, but are not limited to,ethylene glycol, 2,4,7,9-tetramethyldecyne-4,7-diol, poly(propyleneglycol), pentaethylene glycol, tripropylene glycol, triethylene glycol,glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystearate.Examples of aminoalcohol compounds which contain at least one alcoholfunction and one amine function in the same molecule include, but arenot limited to, triisopropanolamine, triethanolamine, ethanolamine,3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, andtetrakis(2-hydroxyethyl)ethylenediamine. The charge adjuvant ispreferably present in the suspending fluid in an amount of about 1 toabout 100 milligrams per gram (“mg/g”) of the particle mass, and morepreferably about 50 to about 200 mg/g.

The surface of the particle may also be chemically modified to aiddispersion, to improve surface charge, and to improve the stability ofthe dispersion, for example. Surface modifiers include organicsiloxanes, organohalogen silanes and other functional silane couplingagents (Dow Corning® Z-6070, Z-6124, and 3 additive, Midland, Mich.);organic titanates and zirconates (Tyzor® TOT, TBT, and TE Series, duPont); hydrophobing agents, such as long chain (C₁₂ to C₅₀) alkyl andalkyl benzene sulphonic acids, fatty amines or diamines and their saltsor quaternary derivatives; and amphipathic polymers which can becovalently bonded to the particle surface.

In general, it is believed that charging results as an acid-basereaction between some moiety present in the continuous phase and theparticle surface. Thus useful materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

Different non-limiting classes of charge control agents which are usefulinclude organic sulfates or sulfonates, metal soaps, block or combcopolymers, organic amides, organic zwitterions, and organic phosphatesand phosphonates. Useful organic sulfates and sulfonates include, butare not limited to, sodium bis(2-ethylhexyl)sodium sulfosuccinate,calcium dodecylbenzenesulfonate, calcium petroleum sulfonate, neutral orbasic barium dinonylnaphthalene sulfonate, neutral or basic calciumdinonylnaphthalene sulfonate, dodecylbenzenesulfonic acid sodium salt,and ammonium lauryl sulfate. Useful metal soaps include, but are notlimited to, basic or neutral barium petronate, calcium petronate, Co-,Ca-, Cu-, Mn-, Ni-, Zn-, and Fe-salts of naphthenic acid, Ba-, Al-, Zn-,Cu-, Pb-, and Fe-salts of stearic acid, divalent and trivalent metalcarboxylates, such as aluminum tristearate, aluminum octanoate, lithiumheptanoate, iron stearate, iron distearate, barium stearate, chromiumstearate, magnesium octanoate, calcium stearate, iron naphthenate, zincnaphthenate, Mn- and Zn-heptanoate, and Ba-, Al-, Co-, Mn-, andZn-octanoate. Useful block or comb copolymers include, but are notlimited to, AB diblock copolymers of (A) polymers of2-(N,N-dimethylamino)ethyl methacrylate quaternized with methylp-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and combgraft copolymers with oil soluble tails of poly(12-hydroxystearic acid)and having a molecular weight of about 1800, pendant on an oil-solubleanchor group of poly(methyl methacrylate-methacrylic acid). Usefulorganic amides include, but are not limited to, polyisobutylenesuccinimides such as OLOA 1200 and 3700, and N-vinylpyrrolidonepolymers. Useful organic zwitterions include, but are not limited to,lecithin. Useful organic phosphates and phosphonates include, but arenot limited to, the sodium salts of phosphated mono- and di-glycerideswith saturated and unsaturated acid substituents.

Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, non-aqueous surfactants may be used. These include, but arenot limited to, glycol ethers, acetylenic glycols, alkanolamides,sorbitol derivatives, alkyl amines, quaternary amines, imidazolines,dialkyl oxides, and sulfosuccinates.

D. Encapsulation

Encapsulation of the internal phase may be accomplished in a number ofdifferent ways. Numerous suitable procedures for microencapsulation aredetailed in both Microencapsulation, Processes and Applications, (I. E.Vandegaer, ed.), Plenum Press, New York, N.Y. (1974) and Gutcho,Microcapsules and Microencapsulation Techniques, Noyes Data Corp., ParkRidge, N.J. (1976). The processes fall into several general categories,all of which can be applied to the present invention: interfacialpolymerization, in situ polymerization, physical processes, such ascoextrusion and other phase separation processes, in-liquid curing, andsimple/complex coacervation.

Numerous materials and processes should prove useful in formulatingdisplays of the present invention. Useful materials for simplecoacervation processes to form the capsule include, but are not limitedto, gelatin, poly(vinyl alcohol), poly(vinyl acetate), and cellulosicderivatives, such as, for example, carboxymethylcellulose. Usefulmaterials for complex coacervation processes include, but are notlimited to, gelatin, acacia, carageenan, carboxymethylcellulose,hydrolyzed styrene anhydride copolymers, agar, alginate, casein,albumin, methyl vinyl ether co-maleic anhydride, and cellulosephthalate. Useful materials for phase separation processes include, butare not limited to, polystyrene, poly(methyl methacrylate) (PMMA),poly(ethyl methacrylate), poly(butyl methacrylate), ethyl cellulose,poly(vinylpyridine), and polyacrylonitrile. Useful materials for in situpolymerization processes include, but are not limited to,polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde;water-soluble oligomers of the condensate of melamine, or urea andformaldehyde; and vinyl monomers, such as, for example, styrene, methylmethacrylate (MMA) and acrylonitrile. Finally, useful materials forinterfacial polymerization processes include, but are not limited to,diacyl chlorides, such as, for example, sebacoyl, adipoyl, and di- orpoly-amines or alcohols, and isocyanates. Useful emulsion polymerizationmaterials may include, but are not limited to, styrene, vinyl acetate,acrylic acid, butyl acrylate, t-butyl acrylate, methyl methacrylate, andbutyl methacrylate.

Capsules produced may be dispersed into a curable carrier, resulting inan ink which may be printed or coated on large and arbitrarily shaped orcurved surfaces using conventional printing and coating techniques.

In the context of the present invention, one skilled in the art willselect an encapsulation procedure and wall material based on the desiredcapsule properties. These properties include the distribution of capsuleradii; electrical, mechanical, diffusion, and optical properties of thecapsule wail; and chemical compatibility with the internal phase of thecapsule.

The capsule wall generally has a high electrical resistivity. Althoughit is possible to use walls with relatively low resistivities, this maylimit performance in requiring relatively higher addressing voltages.The capsule wall should also be mechanically strong (although if thefinished capsule powder is to be dispersed in a curable polymeric binderfor coating, mechanical strength is not as critical). The capsule wallshould generally not be porous. If, however, it is desired to use anencapsulation procedure that produces porous capsules, these can beovercoated in a post-processing step (i.e., a second encapsulation).Moreover, if the capsules are to be dispersed in a curable binder, thebinder will serve to close the pores. The capsule walls should beoptically clear. The wall material may, however, be chosen to match therefractive index of the internal phase of the capsule (i.e., thesuspending fluid) or a binder in which the capsules are to be dispersed.For some applications (e.g., interposition between two fixedelectrodes), monodispersed capsule radii are desirable.

An encapsulation technique that is suited to the present inventioninvolves a polymerization between urea and formaldehyde in an aqueousphase of an oil/water emulsion in the presence of a negatively charged,carboxyl-substituted, linear hydrocarbon polyelectrolyte material. Theresulting capsule wall is a urea/formaldehyde copolymer, whichdiscretely encloses the internal phase. The capsule is clear,mechanically strong, and has good resistivity properties.

The related technique of in situ polymerization utilizes an oil/wateremulsion, which is formed by dispersing the electrophoretic fluid (i.e.,the dielectric liquid containing a suspension of the pigment particles)in an aqueous environment. The monomers polymerize to form a polymerwith higher affinity for the internal phase than for the aqueous phase,thus condensing around the emulsified oily droplets. In one in situpolymerization process, urea and formaldehyde condense in the presenceof poly(acrylic acid) (see, e.g., U.S. Pat. No. 4,001,140). In otherprocesses, described in U.S. Pat. No. 4,273,672, any of a variety ofcross-linking agents borne in aqueous solution is deposited aroundmicroscopic oil droplets. Such cross-linking agents include aldehydes,especially formaldehyde, glyoxal, or glutaraldehyde; alum; zirconiumsalts; and polyisocyanates.

The coacervation approach also utilizes an oil/water emulsion. One ormore colloids are coacervated (i.e., agglomerated) out of the aqueousphase and deposited as shells around the oily droplets through controlof temperature, pH and/or relative concentrations, thereby creating themicrocapsule. Materials suitable for coacervation include gelatins andgum arabic. See, e.g., U.S. Pat. No. 2,800,457.

The interfacial polymerization approach relies on the presence of anoil-soluble monomer in the electrophoretic composition, which once againis present as an emulsion in an aqueous phase. The monomers in theminute hydrophobic droplets react with a monomer introduced into theaqueous phase, polymerizing at the interface between the droplets andthe surrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

Coating aids can be used to improve the uniformity and quality of thecoated or printed electrophoretic ink material. Wetting agents aretypically added to adjust the interfacial tension at thecoating/substrate interface and to adjust the liquid/air surfacetension. Wetting agents include, but are not limited to, anionic andcationic surfactants, and nonionic species, such as silicone orfluoropolymer-based materials. Dispersing agents may be used to modifythe interfacial tension between the capsules and binder, providingcontrol over flocculation and particle settling.

Surface tension modifiers can be added to adjust the air/microcapsuleinterfacial tension. Polysiloxanes are typically used in such anapplication to improve surface leveling while minimizing other defectswithin the coating. Surface tension modifiers include, but are notlimited to, fluorinated surfactants, such as, for example, the Zonyl®series from du Pont, the Fluorad® series from 3M (St. Paul, Minn.), andthe fluoroalkyl series from Autochem (Glen Rock, N.J.); siloxanes, suchas, for example, Silwet® from Union Carbide (Danbury, Conn.); andpolyethoxy and polypropoxy alcohols. Antifoams, such as silicone andsilicone-free polymeric materials, may be added to enhance the movementof air from within the microcapsule layer to the surface and tofacilitate the rupture of bubbles at the coating surface. Other usefulantifoams include, but are not limited to, glyceryl esters, polyhydricalcohols, compounded antifoams, such as oil solutions of alkylbenzenes,natural fats, fatty acids, and metallic soaps, and silicone antifoamingagents made from the combination of dimethyl siloxane polymers andsilica. Stabilizers such as UV-absorbers and antioxidants may also beadded to improve the lifetime of the ink.

E. Binder Material

The binder typically is used as an adhesive medium that supports andprotects the capsules, as well as binds the electrode materials to thecapsule dispersion. A binder can be non-conducting, semiconductive, orconductive. Binders are available in many forms and chemical types.Among these are water-soluble polymers, water-borne polymers,oil-soluble polymers, thermoset and thermoplastic polymers, andradiation-cured polymers.

Among the water-soluble polymers are the various polysaccharides, thepolyvinyl alcohols, N-methylpyrrolidone, N-vinylpyrrolidone, the variousCarbowax® species (Union Carbide, Danbury, Conn.), andpoly(2-hydroxyethyl acrylate).

The water-dispersed or water-borne systems are generally latexcompositions, typified by the Neorez® and Neocryl® resins (ZenecaResins, Wilmington, Mass.), Acrysol® (Rohm and Haas, Philadelphia, Pa.),Bayhydrol® (Bayer, Pittsburgh, Pa.), and the Cytec Industries (WestPaterson, N.J.) HP line. These are generally latices of polyurethanes,occasionally compounded with one or more of the acrylics, polyesters,polycarbonates or silicones, each lending the final cured resin in aspecific set of properties defined by glass transition temperature,degree of “tack”, softness, clarity, flexibility, water permeability andsolvent resistance, elongation modulus and tensile strength,thermoplastic flow, and solids level. Some water-borne systems can bemixed with reactive monomers and catalyzed to form more complex resins.Some can be further cross-linked by the use of a cross-linking reagent,such as an aziridine, for example, which reacts with carboxyl groups.

A typical application of a water-borne resin and aqueous capsulesfollows. A volume of particles is centrifuged at low speed to separateexcess water. After a given centrifugation process, for example 10minutes at 60× gravity (“g”), the capsules 180 are found at the bottomof the centrifuge tube 182, while the water portion 184 is at the top.The water portion is carefully removed (by decanting or pipetting). Themass of the remaining capsules is measured, and a mass of resin is addedsuch that the mass of resin is, for example, between one eighth and onetenth of the weight of the capsules. This mixture is gently mixed on anoscillating mixer for approximately one half hour. After about one halfhour, the mixture is ready to be coated onto the appropriate substrate.

The thermoset systems are exemplified by the family of epoxies. Thesebinary systems can vary greatly in viscosity, and the reactivity of thepair determines the “pot life” of the mixture. If the pot life is longenough to allow a coating operation, capsules may be coated in anordered arrangement in a coating process prior to the resin curing andhardening.

Thermoplastic polymers, which are often polyesters, are molten at hightemperatures. A typical application of this type of product is hot-meltglue. A dispersion of heat-resistant capsules could be coated in such amedium. The solidification process begins during cooling, and the finalhardness, clarity and flexibility are affected by the branching andmolecular weight of the polymer.

Oil or solvent-soluble polymers are often similar in composition to thewater-borne system, with the obvious exception of the water itself. Thelatitude in formulation for solvent systems is enormous, limited only bysolvent choices and polymer solubility. Of considerable concern insolvent-based systems is the viability of the capsule itself; theintegrity of the capsule wall cannot be compromised in any way by thesolvent.

Radiation cure resins are generally found among the solvent-basedsystems. Capsules may be dispersed in such a medium and coated, and theresin may then be cured by a timed exposure to a threshold level ofultraviolet radiation, either long or short wavelength. As in all casesof curing polymer resins, final properties are determined by thebranching and molecular weights of the monomers, oligomers andcross-linkers.

A number of “water-reducible” monomers and oligomers are, however,marketed. In the strictest sense, they are not water soluble, but wateris an acceptable diluent at low concentrations and can be dispersedrelatively easily in the mixture. Under these circumstances, water isused to reduce the viscosity (initially from thousands to hundreds ofthousands centipoise). Water-based capsules, such as those made from aprotein or polysaccharide material, for example, could be dispersed insuch a medium and coated, provided the viscosity could be sufficientlylowered. Curing in such systems is generally by ultraviolet radiation.

Like other encapsulated electrophoretic displays, the encapsulatedelectrophoretic displays of the present invention provide flexible,reflective displays that can be manufactured easily and consume littlepower (or no power in the case of bistable displays in certain states).Such displays, therefore, can be incorporated into a variety ofapplications and can take on many forms. Once the electric field isremoved, the electrophoretic particles can be generally stable.Additionally, providing a subsequent electric charge can alter a priorconfiguration of particles. Such displays may include, for example, aplurality of anisotropic particles and a plurality of second particlesin a suspending fluid. Application of a first electric field may causethe anisotropic particles to assume a specific orientation and presentan optical property. Application of a second electric field may thencause the plurality of second particles to translate, therebydisorienting the anisotropic particles and disturbing the opticalproperty. Alternatively, the orientation of the anisotropic particlesmay allow easier translation of the plurality of second particles.Alternatively or in addition, the particles may have a refractive indexthat substantially matches the refractive index of the suspending fluid.

An encapsulated electrophoretic display may take many forms. Thecapsules of such a display may be of any size or shape. The capsulesmay, for example, be spherical and may have diameters in the millimeterrange or the micron range, but are preferably from about ten to about afew hundred microns. The particles within the capsules of such a displaymay be colored, luminescent, light-absorbing or transparent, forexample.

It will be apparent to those skilled in the art that numerous changescan be made in the specific embodiments of the present invention alreadydescribed without departing from the spirit scope of the invention.Accordingly, the whole of the foregoing description is to be construedin an illustrative and not in a limitative sense.

From the foregoing, it will be seen that the present invention providesfor ready optimization of adhesive compositions used in electro-opticdisplays. The present can use readily available materials to achieve thedesired adjustment of the volumes resistivity of adhesives, and requiresonly conventional apparatus and processing techniques which are familiarto those skilled in the manufacture of electro-optic displays.

What is claimed is:
 1. An electro-optic display comprising first andsecond substrates, and an adhesive layer and a layer of electro-opticmaterial disposed between the first and second substrates, the adhesivelayer having a volume resistivity in the range of 10⁹ to 10¹¹ ohm cm andcomprising a mixture of an adhesive material having a volume resistivityof at least 5×10¹¹ ohm cm and a filler having a volume resistivity notless than 10⁷ ohm cm, the filler being present in the mixture in aproportion above its percolation threshold in the adhesive material. 2.An electro-optic display according to claim 1 wherein the adhesivematerial comprises a pressure sensitive adhesive.
 3. An electro-opticdisplay according to claim 2 wherein the pressure sensitive adhesive hasa volume resistivity of from 10¹² to 10¹⁵ ohm cm.
 4. An electro-opticdisplay according to claim 2 wherein the pressure sensitive adhesive isan acrylate, urethane or silicone-based pressure sensitive adhesive. 5.An electro-optic display according to claim 1 wherein the filler has avolume resistivity of from 10⁸ to 5×10⁹ ohm cm.
 6. An electro-opticdisplay according to claim 1 wherein the filler comprises any one ormore of polyethylene glycol, a polyurethane polymer dispersion, anacrylate polymer, diamond, zirconium oxide, silicon carbide andsodium-doped glass.
 7. An electro-optic display according to claim 1wherein the filler comprises from 3 to 50 percent by weight of theadhesive layer.
 8. An electro-optic display according to claim 1 whereinthe electro-optic medium comprises a rotating bichromal member orelectrochromic medium.
 9. An electro-optic display according to claim 1wherein the electro-optic medium comprises an electrophoretic mediumcomprising a plurality of capsules, each capsule comprising a capsulewall and an internal phase comprising electrically charged particlessuspended in a suspending fluid and capable of moving through the fluidon application of an electric field to the electrophoretic medium. 10.An electro-optic display according to claim 9 further comprising apolymeric binder within which the capsules are held.
 11. Anelectro-optic display according to claim 9 further comprising at leastone electrode disposed between the electro-optic medium and one of thesubstrates, this electrode being arranged to apply an electric field tothe electro-optic medium.
 12. An electro-optic display according toclaim 11 comprising two electrodes disposed on opposed sides of theelectro-optic medium and between the electro-optic medium and the twosubstrates, at least one of the electrodes and the adjacent substratebeing light-transmissive such that the electro-optic medium can beviewed through the light-transmissive substrate and electrode.